Substantially spherical particles of lithium silicates with improved mechanical properties and improved surface quality

ABSTRACT

Substantially spherical particles of lithium silicates with improved mechanical properties and improved surface quality are provided, which are particularly suited for use as breeding material for tritium. The particles are characterised in that they contain additions of tellurium and/or tellurium compounds.

This is a division of application Ser. No. 08/307,693 filed Dec. 29,1994 now U.S. Pat. No. 5,589,427, which is a U.S. national phase ofinternational application PCT/EP93/00671, filed Mar. 19, 1993.

BACKGROUND OF THE INVENTION

The invention concerns substantially spherical particles of lithiumsilicates wherein in particular the mechanical properties aresignificantly improved by means of special additives (up to 5 wt. %tellurium and/or tellurium compounds), without unfavarably influencingthe property of the particle material of releasing tritium upon neutronbombardment, so that said particles can advantageously be used asbreeder material for tritium.

Lithium silicates present one option for direct production in thereactor of the tritium needed in nuclear fusion. The ##EQU1## present inthe lithium silicate thereby traps the neutrons generated in the fusionprocess and transforms itself into helium and tritium in a nuclearreact-ion (breeeding reaction): ##EQU2##

The tritium generated is then used in the reactor in a plasma in thenuclear fusion reaction, whereby it releases neutrons which can then beused for the breeding reaction: ##EQU3##

The ##EQU4## needed for the breeding reaction can be provided in anumber of forms, for example as lithium silicate, whereby lithiumorthosilicate is preferred (viz. Fusion Technology, 1990, page 978).

In the case of solid lithium carrier agents there are a number ofpossibilities of arranging these in a packet bed (viz. E. Proust et al.,2nd Int. Symp. on Fus. Technol., Karlsruhe, Jun. 2-7, 1991), whereby thearrangement in the form of a pebble bed presents only one option.

If the option of a pebble bed is desired as the arrangement for a fusionreactor, the particles can be directly produced from the melt accordingto a known process (spray process, centrifuging process and the like) ina size of up to some 1000 μm.

In production processes whereby spherical particles are directlyproduced from the melt, micro-cracks and cavities are normally observedwhich may affect up to app. 10-20% of the particles produced. Theseparticles are classed as defective, since they only possess littlemechanical capacitance. For example, the maximum pressure capacitance ofsuch particles of lithium orthosilicate particles falls from 8 Newton to2 Newton, when the above-described defects occur (viz. J. Nuc. Mater.,155-57 (1988), 451). Since the defective particles are statisticallydistributed among the faultless particles, the totality of all particlespossesses a considerable variation in mechanical properties. Thepressure test described below serves to assess mechanical capacitance.

If the particles are subjected to constantly changing temperature andpressure strain during use, a part thereof can be destroyed and theirsuitability thus be called into question. This case is given, if theparticles are used as breeding material for tritium in a fusion reactor.Their suitability as breeding material is determined by thethermocycling test described below.

Simulation tests for the temperature and pressure capacitance of apebble bed in the blanket of a fusion reactor conducted with knownspherical lithium silicates show that up to 10% of the particles used donot resist the strain and break (viz. Fusion Technology, 1990, p. 822).Particularly the above-described defective particles are thus affected.

Tests with additions of silicon to the lithium orthosilicate and/orcompounds thereof as well as aluminum and/or aluminum compounds werealready conducted at laboratory level a number of times; they resultedin improvements, but these could only be obtained upon subsequenttreatment under precisely defined conditions (viz. Fusion Technology,1990, p.822). The subsequent treatment consists of heating the pebblesof Li-orthosilicate in a rotating tube furnace to a temperature ofprecisely 1030° C. and then swiftly cooling them again to below 1024° C.At temperatures above 1024° C. the time during which the Si-rich phaseis liquified (viz. FIG. 5) may thereby not exceed 5 minutes, since thepebbles would otherwise fuse with each other. Pebbles which do not reachthe critical conditions so defined do not achieve an improvement oftheir mechanical properties. The realization of these precisely definedconditions on a large scale is therefore extremely difficult.

SUMMARY OF THE INVENTION

The invention proceeds from the problem of optimizing the mechanical andsurface properties of spherical particles of lithium silicates suited asbreeding material for nuclear fusion, which may contain up to 5 weight-%aluminum and/or silicon and/or compounds thereof as additives, and toreduce the number of defective particles to a minimum, whereby thebreeding qualities are, as far as possible, not to be impaired, i.e.,the release of tritium is to remain as unaffected as possible.

The problem is solved according to the invention in that additions oftellurium and/or tellurium compounds are added. The lithium silicateparticles can contain 0.5--5 wt. % tellurium, a tellurium compound orcombinations thereof.

The particles according to the invention can, as such, be produceddirectly from the melt by a known method. Known production methods otherthan direct production from the melt to produce a packet bed may also beused.

The particles used as breeding material in nuclear fusion technology maybe also be present in forms other than globules (powder, lamina, pressedparticles etc.). For this reason, the invention is also not exclusivelyconfined to spheric particles, but includes particles which are to acertain degree non-spheric.

The additions according to the invention have the effect that, on theone hand, the inclination towards formation of micro-cracks and cavitiesin the particles is greatly reduced, and on the other, their surfaceroughness is substantially diminished. As a result, less variations inthe mechanical and physical properties of the particles are observed andtheir mechanical strength is generally improved to an unexpected extent,as is shown by the comparative tests below.

It is therefore irrelevant whether lithium silicates with enriched ordepleted ##EQU5## are used.

These advantageous properties are maintained even if the particles aresubjected to tempering after the production process.

The mechanical properties of the lithium silicates according to theinvention are so good that they can also be used in other fields oftechnology, e.g., in ball bearings and the like. Spherical particles oflarger dimensions than those used in nuclear fusion are, of course,employed for such applications.

Methods for executing the invention

EXAMPLE

10 pebbles respectively having a particle size of 0.5 mm of samples A toD of known composition serving for comparison and of samples E and F ofthe composition according to the invention are subjected to the pressuretest described below.

The samples were produced directly from the melt. For this purpose,batch mixtures of the compounds of the material components known fromthe prior art (e.g.) Li₂ CO₃, SiO₂, TeO₂) are produced and melted incorrosion-resistant melt vessels at 1380° C.-1400° C. To introduce theLi-content to produce the batch mixtures, compounds are suitablyselected which contain a certain amount of the isotope ⁶ Li, whereby noinfluence can be determined on the melting behavior and structuring ofthe material of the amount of the lithium isotope ⁶ Li in the basematerial. It is, however, also possible to select lithium compoundswherein the ⁶ Li-content corresponds to the normal content correspondingto commercial custom. The production of the pebbles is effected by meansof atomization of the glass flow emitted according to the processdescribed in the US-PS 3,294,511, whose disclosure is referred to. Theemitted liquid melt flow thereby has a temperature of at least 1370°C.-1400° C. and a diameter of 1 to 5 mm. In contrast to the above-citedpublication, a cold compressed air stream is used to atomize the meltflow, whereby the velocity of the compressed air stream is between 50m/s and 300 m/s at the escape point. A temperature of between 30° C. and500° C. prevails in the atomization chamber. The different materialcompositions specified in the table were obtained by means ofcorresponding batch mixture compositions.

Samples A to F described below were melted as raw batch mixture at thetemperatures specified above and the melt thus obtained was then alsosubjected to the atomization treatment described above.

The reference samples A and B consisted of pure lithium orthosilicate, Cand D of lithium orthosilicate additionally containing 2.2 weight-partsSiO₂ per 100 weight-parts.

The samples E and F according to the invention consisted of lithiumorthosilicate additionally containing 2.2 weight-parts SiO₂ and 0.5weight-parts Te per 100 weight-parts.

The samples were subjected to the pressure test both in untreated state(samples A, C and E) and after tempering in a rotating tube furnace(samples B, D and F).

The mechanical stability of the particles was tested with an apparatusspecifically constructed for this purpose, which is shown in FIG. 1. TheLi-orthosilicate pebble is thereby pressed against a carrier mounted onthe weigh bin of a microbalance by means of a piston. The microbalanceshows the real weight imposed on the pebble during the test. By feedingwater into the container to which the piston is attached the pressurebearing on the pebble is gradually raised until the pebble breaks. Thisfracture load acts as the measure of the mechanical stability of thepebbles.

The results of the pressure test are shown in Table 1. They show thatthe pebbles of the composition according to the invention display asignificantly reduced variation of mechanical properties even withouttempering, whereby the fracture load was surprisingly increased by up to30%.

To test the suitability of the particles as breeding material fortritium, the particle beds were subjected to the so-called thermocyclingtest which simulates the thermal and mechanical strain in the blanket ofthe fusion reactor. The particles are thereby filled into a container indense package under vibration, whereby said container comprises twothermocouple elements, of which one is arranged in the center of theparticle bed and the other on the wall of the container (FIG. 2). Thecontainer is induction-heated during the test until the wall reaches atemperature of 600° C. This temperature is maintained until thetemperature in the center of the particle bed reaches a value between530° and 600° C. The container is then quenched in water, so that thetemperature in the container wall sinks to below 100° C. This is themoment in which the particle bed is subjected to the highest mechanicaland thermal strain, since the temperature in the center of the particlebed is still app. 500° C. The tests are first conducted manually for 10cycles using the container according to FIG. 2 and are later continuedin an automatic apparatus (FIG. 3) permitting a greater number ofcycles. A typical temperature curve during the test is shown in FIG. 4.

The results of the thermocycling test are shown in Table 2.

They demonstrate that the spherical particles according to the inventionare more resistant to the mechanical and thermal strain prevailingduring thermocycling of the reactor in the blanket drums than theparticles according to the prior art, both in original state and aftertempering in the rotating tube furnace, whereby in the first case thepercentage improvement compared to reference samples containing SiO₂ isparticularly striking.

It was demonstrated by means of neutron irradiation that the release oftritium from the particles was not affected by the addition of 0.5weight-% Te.

                  TABLE 1                                                         ______________________________________                                        Loads (N) under which the individual pebbles                                  (0.5 mm diameter) broke and fracture loads FL (N)                             (mean values of 10 measurements)                                              (Pebbles of pure lithium orthosilicate resp.                                  lithium orthosilicate + additives)                                            ______________________________________                                                Test No.                                                              Sample    1       2       3    4     5     6                                  ______________________________________                                        A pure, o 2.7     2.7     2.7  2.7   3.5   3.9                                B pure, d 1.6     5.4     3.6  2.5   0.6   0.2                                C +SiO.sub.2, o                                                                         3.5     5.7     11   11    6.2   5.0                                D +SiO.sub.2, d                                                                         7.0     4.8     8.5  9.8   6.9   10                                 E +SiO.sub.2, Te, o                                                                     12      13      7.0  6.0   8.0   12                                 F +SiO.sub.2, Te, d                                                                     5.0     11      9.5  12    10    10                                 ______________________________________                                                Test No.                                                              Sample    7        8        9      10    FL                                   ______________________________________                                        A pure, o 2.5      1.8      3.3    3.6   2.9                                  B pure, d 3.0      3.2      4.3    3.8   2.8                                  C +SiO.sub.2, o                                                                         3.4      12       5.4    9.0   7.2                                  D +SiO.sub.2, d                                                                         6.3      9.0      7.0    7.9   7.7                                  E +SiO.sub.2, Te, o                                                                     13       11       11     13    10.6                                 F +SiO.sub.2 Te, d                                                                      10       8.9      11     11    9.8                                  ______________________________________                                         o = original (as produced)                                                    d = tempering in the rotating tube furnace                                    addition of SiO.sub.2 = 2.2 weight%, of Te = 0.5 weight%                      Samples A to D: prior art; samples E and F according to the invention    

                  TABLE 2                                                         ______________________________________                                        Results of the simulation of mechanical and thermal                           tension in the blanket (thermocycling test) with                              pebbles of 0.45-0.56 mm diameter                                              Material Composition   % break % micro-content*                               ______________________________________                                        86/1 o (V)                                                                             +2.2 weight-% SiO.sub.2                                                                     11      0.02                                           86/1 a (V)                                                                             "             2       0.02                                           89/1 o (V)                                                                             Li.sub.4 SiO.sub.2                                                                          6       0.12                                           89/1 a (V)                                                                             "             10      0.05                                           90/1 o (V)                                                                             +2.2 weight-% SiO.sub.2                                                                     6.0     **                                             90/1 d (V)                                                                             +2.2 weight-% SiO.sub.2                                                                     1.4     **                                             90/5 o (E)                                                                             +2.2 weight-% SiO.sub.2                                                       +0.5 weight-% Te                                                                            4.2     **                                             90/5 d (E)                                                                             "             1.2     **                                             ______________________________________                                         *particle < 0.05 mm                                                           **negligible                                                                  o = as produced                                                               d = tempered in rotating tube furnace                                         a = tempered in stationary furnace                                            V = comparison                                                                E = according to the invention                                           

The differences of the test results for reference samples 86/1 and 90/1result from the improved production technology of the latter compared tothe prior art.

In the reference sample without an excess of SiO₂ (89/1) the temperingresulted in a deterioriation of the mechanical properties due toenhanced crystal formation, as shown in Table 2.

We claim:
 1. A process for making lithium orthosilicate particlescomprising producing lithium orthosilicate particles from a meltcomprising a lithium compound and a silicon compound, said melt furthercontaining tellurium, a tellurium compound, or combinations thereof,said melt further optionally containing aluminum, an aluminum compoundor mixtures thereof.
 2. A process according to claim 1, wherein saidlithium orthosilicate particles contain up to 5 wt. % of tellurium, atellurium compound or mixtures thereof.
 3. A process according to claim1, wherein said lithium orthosilicate particles contain up to 5 wt. % ofaluminum, an aluminum compound, silicon, or a silicon compound ormixtures thereof.
 4. A process according to claim 2, wherein saidlithium orthosilicate particles contain up to 5 wt. % of aluminum, analuminum compound, silicon, or a silicon compound or mixtures thereof.5. A process according to claim 2, wherein said particles aresubstantially spherical.
 6. A process according to claim 4, wherein saidparticles are substantially spherical.
 7. A process according to claim2, wherein said lithium orthosilicate particles contain 0.5-5 wt. %tellurium, a tellurium compound or mixtures thereof.
 8. A processaccording to claim 3, wherein said lithium orthosilicate particlescontain 0.5-5 wt. % tellurium, a tellurium compound or mixtures thereof.9. A process according to claim 3, wherein said lithium orthosilicateparticles contain up to 2.2 wt. % SiO₂.
 10. A process according to claim4, wherein said orthosilicate particles contain up to 2.2 wt. % SiO₂.11. A process according to claim 8, wherein said lithium orthosilicateparticles contain up to 2.2 wt. % SiO₂.
 12. A process for making lithiumorthosilicate comprising:producing a melt by melting (a) at least onelithium compound, (b) at least one silicon compound, (c) tellurium, atellurium compound, or combinations thereof, and optionally (d)aluminum, an aluminum compound or mixtures thereof; and atomizing saidmelt to form lithium orthosilicate particles containing tellurium, atellurium compound or combinations thereof.
 13. A process for performinga breeding reaction for the generation of tritium to be used in anuclear fusion reaction, said process comprising bombarding a lithiumorthosilicate breeding material with neutrons, whereby lithium istransformed into helium and tritium, the improvement wherein saidlithium orthosilicate breeding material is lithium orthosilicateparticles containing tellurium, a tellurium compound, or combinationsthereof.
 14. A process according to claim 13, wherein said lithiumorthosilicate particles contain up to 5 wt. % of tellurium, a telluriumcompound or mixtures thereof.
 15. A process according to claim 13,wherein said lithium orthosilicate particles contain up to 5 wt.% ofaluminum, an aluminum compound, silicon, or a silicon compound ormixtures thereof.
 16. A process according to claim 14, wherein saidlithium orthosilicate particles contain up to 5 wt.% of aluminum, analuminum compound, silicon, or a silicon compound or mixtures thereof.17. A process according to claim 13, wherein said particles aresubstantially spherical.
 18. A process according to claim 13, whereinsaid lithium orthosilicate particles contain 0.5-5 wt. % tellurium, atellurium compound or mixtures thereof.
 19. A process according to claim14, wherein said lithium orthosilicate particles contain up to 2.2 wt. %SiO₂.